Suspensions are encountered in many types of industrial and/or consumer products, such as pharmaceutical products, alimentary products, detergent products, personal care products, paints and coatings, inks, fluid construction materials, agricultural products, etc.
A suspension, generally speaking, is a mixture of a liquid and a plurality of particles, typically of a solid material. Usually the suspension particles are larger than 1 micrometer in diameter (visible under a microscope). The size of the suspension particles typically results in the suspension being thermodynamically unstable; suspensions will settle over time if left undisturbed. This distinguishes a suspension from a colloid, in which the particles are smaller and do not settle.
It is generally understood that particle radius and rheology characteristics (of the continuous phase) have a bearing on suspension stability. This is expressed in the Stokes equation, which can be used to predict settling velocity, with continuous phase viscosity and particle radius as the main variables.
Similar considerations apply to the stabilization of gas bubbles suspended in a liquid, e.g. to produce a (low gas/liquid ratio) liquid foam, where gas bubble radius and rheology characteristics will determine whether the system is stable or whether the gas bubbles have a tendency for upward flotation, to eventually escape from the system.
It is known in the art to use additives for increasing the continuous phase viscosity in order to stabilize suspended solid particles or gas bubbles. To assure stability of such a system when it is left undisturbed additives have to be selected that confer a high viscosity at low or no shear, a property characterized by the zero-shear and/or low-shear viscosity. Some additives may confer a yield stress, meaning that a critical level of stress must be applied to initiate flow. The capability to stabilize suspended solid particles or gas bubbles in principle is characterized by the yield stress value and/or a sufficiently high zero-shear and/or a sufficiently high low-shear viscosity value.
An important factor in stabilization of suspended solid particles or gas bubbles is that it should not prohibit the product from flowing (without having to apply excessive forces), e.g. to extract it from a container or dispenser. Specific types of applications may require specific flow behavior. Ideally, for many applications, an additive for stabilization of suspended solid particles or gas bubbles should show shear thinning behavior, meaning that the viscosity is reduced when shear is increased to provide flowing behavior, while the viscosity at rest or at small disturbances can be high enough to stabilize the suspension. Shear thinning capabilities may be characterized by the pouring viscosity and the ratio of the pouring viscosity and low-stress viscosity values.
The ability of a certain structuring agent to provide shear thinning behavior alone is not necessarily sufficient to determine whether the liquid product is capable of suspending particles or gas bubbles with sufficient stability (and vice versa). For example, excessive amounts of additive may provide the required degree of stabilization but result in the liquid composition becoming overly viscous and non-pourable.
It is also common ground that stabilization benefits should be attained at as low a level of additive as possible for cost and formulation considerations, as is the case with any additive.
The development of additives that can stabilize systems comprising suspended particles or gas bubbles by retarding or preventing migration of the suspended solid particles or gas bubbles, without (negatively) affecting other properties of a formulation, in particular flow behavior, therefore remains technologically challenging.
Aiding to this challenge are current trends towards the use of bio-based ingredients, to reduce the environmental impact of the products in the broadest sense, and towards more concentrated products.